Add Pinwheel Clause support for zone-based encounters in route groups

Allows each sub-zone within a route group to have its own independent
encounter when the Pinwheel Clause rule is enabled (default on), instead
of the entire group sharing a single encounter.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/app/seeds/data/black-2.json

@@ -1333,6 +1333,7 @@
       {
         "name": "Pinwheel Forest (Outside)",
         "order": 23,
+        "pinwheel_zone": 1,
         "encounters": [
           {
             "pokeapi_id": 533,
@@ -1379,6 +1380,7 @@
       {
         "name": "Pinwheel Forest (Inside)",
         "order": 24,
+        "pinwheel_zone": 2,
         "encounters": [
           {
             "pokeapi_id": 550,